Olefins, particularly C2 and C3 olefins, are desirable as a feedstock for making derivative products such as oligomers, e.g., higher olefins, and polymers such as polyethylene and polypropylene. Traditionally, olefin feedstock was produced by cracking petroleum feedstock. An olefin plant has at least two parts. The reactor section makes the olefins. The recovery section recovers the desired olefins and removes impurities. The object of the recovery section is to recover olefins that meet the proper grade specification in the most efficient way.
U.S. Pat. No. 5,090,977 discloses a process for making olefins by steam cracking. The process includes separating the olefin product into methane, hydrogen, ethane, ethylene, propylene and C5+ streams. The disclosed separation preferentially produces propylene. No propane, butane, butylene, or butadiene streams are produced.
Use of an oxygenate feedstock is becoming an alternative to petroleum feedstock for making olefins, particularly large quantities of ethylene and propylene for the production of higher olefins and plastic materials. In general, the olefins are formed by contacting the oxygenate components with a molecular sieve catalyst to convert oxygenates to olefins. The oxygenate-to-olefin process also needs a recovery section to separate the desired olefins from byproducts. The separation process of an oxygenate-to-olefin process needs to take into account the unique composition of the oxygenate-to-olefin reactor effluent.
U.S. Pat. No. 6,121,504 discloses a process for making olefin product from oxygenate feedstock using molecular sieve catalysts. Water and some unwanted byproducts are removed from the olefin product by contacting with a quench medium. After contacting with the quench medium, an olefin product fraction is obtained which comprises the desired olefins, but also includes dimethyl ether, methane, CO, CO2, ethane, propane, and other components such as water and unreacted oxygenate feedstock. The methanol feed is reboiled by the quench medium. However, there is no discussion of alternative ways of using heat from the quench bottoms.
U.S. Pat. No. 4,474,647 discloses that dimethyl ether can adversely impact the oligomerization of certain olefins. The patent describes a process for removing dimethyl ether from a C4 and/or C5 olefin stream using distillation. The stream is distilled and separated into an overhead and a bottoms stream. The overhead stream contains dimethyl ether, water, and various hydrocarbons, and the bottoms stream contains purified olefins.
Eng et al., “Integration of the UOP/HYDRO MTO Process into Ethylene Plants, ” 10th Ethylene Producers' Conference, 1998, disclose a de-ethanizer first flow scheme process. The flow diagram discloses a quench tower and an oxygenate removal unit. However, there is no discussion of how heat is used from the quench. EP-B1-0 060 103 discloses a process for extracting dimethyl ether from a vapor stream containing ethylene and propylene using a methanol wash system. The methanol wash removes a substantial amount of the dimethyl ether, but also removes a significant amount of the ethylene and propylene. This patent likewise does not disclose how the heat can be used more efficiently.
U.S. Pat. No. 6,403,854 discloses a method for heating the reboiler from a propane/propylene splitter with water from a quench tower in the recovery section of an oxygenate to olefin reactor. The propane/propylene splitter is located after all the oxygenates are removed from the effluent stream.
Additional methods of removing undesirable components from olefin streams are sought. In particular, it is desirable to have better methods for removing byproducts such as propane, dimethyl ether, methyl acetylene, propadiene, acetaldehyde and other byproducts without a significant loss of olefin product. It would also be better if the resulting process was heat-integrated to maximize efficiency. The present invention satisfies these and other needs.